The invention disclosed herein pertains to enhancing the visibility of indicator lamps in display panels, particularly, when the indicators are viewed from a substantial distance and at a small angle relative to the plane of the display panel.
Display panels are widely used to indicate to an attendant the state or condition of, for example, a machine part, various electronic circuits, limit conditions, on and off states, and so forth. Such display panels are sometimes mounted on one or more machines which are under the control of a single operator or they may be installed at control stations or consoles where a single operator must observe or respond to indicator lamps associated with the panels turning on or off. One well known type of display panel comprises a transparent substrate or sheet of plastic which has windows, (not apertures) developed on them by printing or otherwise coating the back side of the panel with ink to provide a background color and masking a plurality of areas so that these areas constitute windows. Usually, the substrate or panel sheet is treated on one side with ink as received from the supplier or after having been received which, when cured, results in the rear surface of the sheet assuming a textured appearance. In other words, the sheet becomes translucent rather than transparent. As a result, the window areas are sometimes simply translucent and uncolored whereas in other cases, as a matter of choice, some various colors can be applied to the backside so that some of the windows may have a particular color and other of the windows may have other colors.
Generally, there will be an on/off light source such as an incandescent lamp or more usually a light emitting diode (LED) behind each window. LED's are used in most panels today because they consume less electric power than incandescent indicator lamps. LEDs are available in a variety of colors such as red, orange, green and amber. Almost invariably, there are legends or other graphics printed on the rear side of the panel for being visualized from the front adjacent an associated indicator lamp window. Typically, when a window becomes illuminated as a result of the indicator LED behind it having turned on, the operator-attendant is required to take some action or simply note the indication.
The operator-attendant can usually observe a large number of indicator windows in several panels in the console within a single viewing angle. In other words, when the operator-attendant needs to be concerned only with viewing indicator windows to which the optical axes of the viewers eyes are perpendicular or nearly perpendicular to the plane of the panel, the viewer will have no difficulty in perceiving if the indicator LED is on or off. However, in consoles that have indicator panels extending a substantial distance to the left and right of where the viewer is ordinarily situated, the viewer will not be sure if a window is illuminated or dark. So the viewer must move laterally to obtain more direct alignment with the indicator windows being surveyed in order to be sure if the indicator window is illuminated or not illuminated. A common complaint of those who attend a multiplicity of display panels is that they cannot ascertain if a window is illuminated or not until they move bodily to allow their optical axes to become more nearly perpendicular to the panel. In some cases, such as at a nuclear power plant control console, the inability of the operator-attendant to know when one or more distant indicator windows are illuminated such that an action should be undertaken, may result in severe damage to persons or equipment.